<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title></title>
  <style>
    body {
      padding: 0;
      margin: 0;
    }

    .letter {
      background-color: cadetblue;
      color: #fff;
      padding: 5px;
      position: sticky;
      top: 0;
    }

    .content {
      height: 200px;
      padding: 10px;
    }

    .more {
      position: fixed;
      right: 10px;
      top: 100px;
      user-select: none;
    }

    .more-latter.active {
      color: red
    }
  </style>
</head>

<body>
  <div class="letter" id="lettera">A</div>
  <div class="content">xxxxxxxxxxxx</div>
  <div class="letter">B</div>
  <div class="content">xxxxxxxxxxxx</div>
  <div class="letter">C</div>
  <div class="content">xxxxxxxxxxxx</div>
  <div class="letter">D</div>
  <div class="content">xxxxxxxxxxxx</div>
  <div class="letter">E</div>
  <div class="content">xxxxxxxxxxxx</div>
  <div class="more">
    <div class="more-latter">A</div>
    <div class="more-latter">B</div>
    <div class="more-latter">C</div>
    <div class="more-latter">D</div>
    <div class="more-latter">E</div>
  </div>
</body>

</html>